South Africa has large coal reserves and produces approximately 74% of its primary
energy from coal. Coal gasification using moving bed gasifiers is one of the most
important coal utilisation technologies, consuming ± 17.5% of locally produced coal.
This study was motivated by the need to investigate alternative coal gasification
technologies for the utilisation of fine, high-ash and caking coals for future Integrated
Gasification Combined Cycle (IGCC) and coal to liquids (CTL) plants. These coals
are estimated to form a large percentage of the remaining coal reserves in South
Africa and could be difficult to utilise efficiently in moving bed gasifiers.
Fluidised bed gasification was identified as a technology that could potentially utilise
these coals. Coals from the New Vaal and Grootegeluk collieries were selected as
being suitable for this investigation. The coals were subjected to detailed
characterisation, bench-scale and pilot-scale fluidised bed gasification tests.
The results of the pilot-scale atmospheric bubbling fluidised bed gasification tests
show that stable gasification is possible at temperatures between 880 °C and 980 °C.
The maximum fixed carbon conversion achievable in the pilot plant is, however,
limited to ± 88% due to the low reactivity of the coals tested and to thermal
fragmentation and attrition of the coal in the gasifier. It was found that oxygen
enrichment of the gasification air from 21% to 36% by means of oxygen addition
produces a significant increase in the calorific value of the gas (3.0 MJ/Nm3 to
5.5 MJ/Nm3). This observation has not previously been reported at pilot-plant scale.
A mathematical model for a bubbling fluidised bed coal gasifier was developed based
on sub-models for fluidised bed hydrodynamics, coal devolatilisation, chemical
reactions, transfer processes and fines generation. A coal devolatilisation sub-model
to predict the products of coal devolatilisation in a fluidised bed gasifier was
developed and incorporated into the model. Parameters associated with the rates of
the gasification reactions and the devoltilisation process were obtained by means of
bench-scale tests. The heat loss parameter (Q) in the model was estimated by means
of a heat loss calculation.
The results from the pilot-scale gasification tests were used to evaluate the predictive
capability of the model. It was found that for temperature, fixed carbon conversion
and calorific value of the gas the difference between measured and predicted values
was less than 10%. Recommendations are made for further refinement of the model to
improve its predictive capability and range of application.
The model was used to study the effect of major operating variables on gasifier
performance. It was found that increasing the reactant gas (air, oxygen and steam)
temperature from 250 °C to 550 °C increases the calorific value of the gas by ± 9.3%
and the gasification efficiency by ± 6.0%. Increasing the fluidised bed height has a
positive effect on fixed carbon conversion; however, at higher bed heights the benefit
of increasing the bed height is less due to the inhibiting effects of H2 and CO on the
rates of char gasification. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2014
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/15214 |
| Date | January 2014 |
| Creators | Engelbrecht, André Daniël |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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